Re: More facts: Coil size to faraday cage size ratio

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi Fin

Its probably cheaper and less demanding on the each cage to have a double
one.
Two poor performance cages can be more effective than one good one and
possibly two are essential if the inner is going to be in the discharge
path.
There should be at least one line filters in the cage wall.

If you have not done it already minimized  at the source with "good
practice"
i.e. a symmetrical primary circuit with minimized loops and shielded
preferably.
Even the primary coil can be made symmetrical with a double layer and even
shielded to some extent with strike rails top and bottom.
Perhaps the primary connection would run up a metal bar that grounds the
strike rails and secondary to the base.
Use a metal bottom base bonded at each corner to the ground plane of your
cage
Operate with the lowest coupling, shortest secondary and biggest top load
you can get away with.
Perhaps the SG could be low pass filtered to damp the spurious resonances of
the primary.
Consider a bipolar or a twin
If you want a really clean coil go solid state.

Bob



----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Monday, November 22, 2004 3:20 PM
Subject: More facts: Coil size to faraday cage size ratio


> Original poster: "Finn Hammer" <f-h@xxxx>
>
> I have some more facts of the measurements made on the coil.
>
> The purpose of the measurements was to get a rough idea of what kind of
> shielding would be needed to comply with rules.
> The measurements were made with a bi-conical antenna, looking much like 2
> egg-beaters combined at the shafts. The frequency responce of the antenna
> is 30-300MHz, and the signal is fed into a HP spectrum analyzer type
> 8591EM. This instrument is corrected for the responce of the antenna, and
> this is the reason for the apparent dip in radiated level on these curves,
> where the first curve is the signal from the coil, the second is the
> background noise.
> http://home5.inet.tele.dk/f-hammer/emc.pdf
> The frequency scale is linear, the level is a dB scale.
> The standard that we use as reference is a ISM standard, EN55011,
> Industrial,scientific, medical,  line, radiated, equipment (I didn`t get
> the rest)
> The guys that I work with say, that pulsed equipment is given an
advantage,
> but that it is perhaps about 10dB, and not something to be concerned about
> at this moment.
>
> We could pass DC into the cages, and feed it into a swichmode converter,
> and we are investigating the feasibility of this solution.
>
> We realize that a dual cage is probably needed, and are going o make a
> prototype cage this way.
>
> Steve, the shielded pulse transformer you mentioned is definately going to
> be needeed, so I`l appreciate your making one.
>
> I will keep you all informed in the future of the development of this
cage,
> as well as all measurements when they are available.
>
> Cheers, Finn Hammer
>
> Tesla list wrote:
>
> >Original poster: "Bert Hickman" <bert.hickman@xxxxxxxxxx>
> >Hi Finn,
> >
> >Was the measurement made using a shielded loop or a dipole antenna, and
at
> >what frequency range(s), and at what distance(s)? In any event, the EMI
> >measurement was made within the "near field" (a distance less than
> >Lambda/2*Pi from the source). And, although it's likely that your current
> >Faraday cage may work fairly well for damping the E-field around your
> >coil, unless your shield uses high Mu material the near field magnetic
> >induction field is probably passing right through your Faraday shield.
> >
> >The low frequency near-field magnetic component can be a very significant
> >problem, especially when dealing with equipment that uses low frequency
> >(<200 kHz) and high current loops (i.e., induction heating, air core RF
> >transformers, and SSTC's). The problem becomes worse as you lower the
> >operating frequency. Fortunately, the near H-field declines as the cube
of
> >the distance, so doing the EMI measurement at a further distance may help
> >bring you closer to spec... but probably not by a factor of 100. If the
> >EMC test that failed used a loop antenna, you were picking up the H
> >induction field. You'll probably need to use magnetic shielding material
> >in your Faraday cage in order to effectively shield it.
> >
> >Best regards,
> >
> >-- Bert --
>
>
>
>
>